This invention relates generally to double base propellant compositions and more particularly to energetic plasticizers for double base propellants.
Solid propellants can be classified as being either homogeneous or composite. The former refers to those types considered true monopropellants in which each molecule contains all the necessary fuel and oxygen for combustion. The composite type propellant, in contrast, consists of a physical mixture of a fuel and an oxidizer. The homogeneous propellants are further subclassified as being either single or double base depending on whether the composition contains a single energetic combustible or contains an additional energetic combustible or a mixture of additional energetic combustibles which act as an energetic plasticizer for the first energetic combustible. Throughout this specification the first energetic combustible is referred to as the polymer, the second energetic combustible as the energetic plasticizer, and the combination of the two as the binder.
Recently, additional oxidizers and metal fuels have been added to double base propellants. If an additional oxidizer(s) is added, the double base propellant is referred to as a modified double base propellant. A double base propellant with an additional oxidier(s) and additional metal fuel(s) is termed composite modified double base propellant. These double base propellants can be considered as hybrids between the homogeneous and composite propellants.
The term plasticization is synomous with gelatinization. It is used to describe the initial physico-chemical reaction of the polymer with the energetic plasticizer. By this reaction, the polymer and the energetic plasticizer form a soft colloidal dispersion. This dispersion is then made usable for propellant purposes by the application of mild heat over a period of time to form a tough, elastic, rubbery solid. This reaction is referred to as curing the propellant composition.
If the polymer is not plasticized by energetic plasticizers, solvents must be used in the processing in order to obtain the colloidal dispersion. Solvent processes always impart a varying amount of residual volatiles in the propellant which cause the energy content to vary accordingly. Further, solvent processes require an objectionally long drying cycle and are not practical for forming most rocket propellant charges.
However, the use of solventless double base propellants has been declining in recent years because of a lack of alternative energetic plasticizer for nitroglycerin. Its extreme sensitivity and high energy make nitroglycerin dangerous to handle and make the resulting propellant composition more sensitive to unwanted detonation. Because of this sensitivity nitroglycerin propellants require a high hazard classification which means extra expense for storage, and a more limited reserve. Other problems arise from the vapors of nitroglycerin. They cause sickness and headaches to humans, thereby causing difficulties in the manufacture, handling, and storage of any nitroglycerin composition. Another troublesome characteristic of nitroglycerin is its tendency to migrate out of composition. This results in a shorter shelf life and a poorer firing accuracy due to variance in propellant energy. Also flame temperatures of a propellant containing nitroglycerin are high. If the propellant is to be used in guns, this characteristic necessitates the addition of coolants which may produce soot and smoke in the exhaust. If coolants are not used, the erosion of the gun barrel is appreciably increased.
Accordingly, much research has been expended since World War II to find alternative energetic plasticizers, or at least a partial substitute for nitroglycerin. Unfortunately the resulting compositions had poor performance or poor mechanical strength or had to rely excessively on solvents for processing. Also the energetic plasticizers other than nitroglycerin had an appreciably lower density and lower energy content or had poor plasticizing capacity for nitrocellulose and other polymers, or were extremely expensive.
Polyolpolynitrates have attracted much attention by researchers. Many of these nitroxy compounds have an energy content, density, and cost comparable to nitroglycerin. But on contact with the various fibrous polymers, very little plasticization occurs. On the other hand, many other polyolpolynitrate plasticize nitrocellulose, but they have a poor energy content.
Homologs of sorbitol nitrate are polyolpolynitrates. Due to the difficulty in preparing these homologs in their pure form, their use in the propellant and explosive arts has been as a mixture of homologs consisting of a large preponderance of the hexa homolog, a small amount of the penta homolog, and traces of the other homologs. These previous mixtures were unable to plasticize nitrocellulose or any other polymer used in propellant and they crystalized easily.
The advantages of the tetra-,penta-, and hexa-sorbitol nitrate mixtures are significant. Their molecules are large and capable of hydrogen bonding. Hence there is little tendency for sorbitol nitrates to migrate out of the propellant composition. Further, the molecular arrangement gives a high density plasticizer, actually as much as 4% denser than NG. These advantages combined with the high energy content and low cost make mixtures of sorbitol nitrates an excellent energetic plasticizer if the plasticizing capacity of this mixture is increased to a practical level.